While the present invention finds particular usefulness in the oil industry, it has particular application in other hostile pressure and temperature environments where size and accuracy are important. In an oil well pressure transducer, it is common to size a unit to go into a small diameter metal pressure housing for measurement of pressures up to 20,000 psi and even more and which can be exposed to downhole temperatures up to 400.degree. F. or more. The pressure housing must have a wall thickness sufficient to withstand the downhole pressures so that the O.D. of the internal pressure transducer is in the neighborhood of one inch.
A downhole oil well pressure gauge can be transported by a wireline, cable or pipe string to one or more levels of interest in a well bore where both temperature and pressure are sensed over a period of time. Typically, pressure measurements are repeatedly sampled and recorded over a period of time at a sampling rate determined by down hole electronics and may be stored in a downhole memory for subsequent replay or sent to the surface for analysis. Alternately, sometimes gauges are attached to production strings or other downhole equipment for extended periods of time or "permanently". In both uses of the gauge, sudden downhole pressure changes can also typically be accompanied by a temperature change in a relatively short period of time. It is also important for the accuracy of the pressure sensor to not change its calibration over a period of time in the borehole in response to pressure or temperature effects in the boreholes. Thus, there is a need for a pressure gauge for high pressure measurements which is also insensitive to sudden changes in temperature or effects of pressure. The present invention relates to high pressure transducers which can accurately measure high pressure changes under transient temperature conditions. The invention is also applicable to low pressure designs.
Under the combined effect of high temperature and pressure conditions, the typical pressure sensor structure is subjected to high stress by the applied high pressure and subjected to high temperature both of which cause creep in the materials. Creep in materials tends to be exponentially related to both temperature and stress levels. The effect of creep or permanent deformation is to alter the calibration or measurement characteristics of a sensor and cause the sensor to obtain inaccurate measurements from its calibration standard over a period of time.
Heretofore, capacitance type transducers have been utilized for pressure measurements where a downhole oil well pressure varies an electrical capacitance as a function of pressure. The pressure is translated to a capacitance measurement by a pressure diaphragm moving parallel arranged capacitance plates toward and away from one another. An example of this kind of device is shown in U.S. Pat. No. 4,322,775.
I have also coupled a bourdon tube to a capacitance type of sensor as disclosed in U.S. Pat. No. 4,873,870 in which sensor system, the pressure in the bourdon tube generates a directional linear force to displace quartz supported parallel arranged capacitor elements toward and away from one another. While this device is satisfactory for a number of applications, it is a difficult unit to manufacture.
In the present invention, I have developed a capacitor device which has a unique relationship of capacitors and can be constructed from metals to respond to low force inputs and be relatively insensitive to temperature changes. The capacitor device is particularly adaptable to high pressure measurements with a high degree of accuracy.